Product Code: ICAL07_103

Liquation Mechanism Analysis on Laser Deposition on Directionally Solidified Superalloy Blade for Crack-free Rejuvenation
Authors:
Minlin Zhong, Tsinghua University; Beijing Peoples Republic of China
Wenjin Liu, Tsinghua Univ.; Beijing Peoples Republic of China
Xiaofeng Zhu, Tsinghua University; Beijing Peoples Republic of China
Qian Hao, Tsinghua University; Beijing Peoples Republic of China
Presented at ICALEO 2007

Development of cost-effective repair and rejuvenation processes for Directionally Solidified (DS) Ni-based superalloy blades are of great economic and technological meanings. Laser deposition is a promising rejuvenation candidate for DS blades due to its precise controllability on heat input, solidification conditions, deposition geometry and microstructure. However, one of the challenges for rejuvenating DS blade is the extremely high liquation and cracking susceptibility. Boundary liquation and interface cracking frequently occur during laser deposition on the DS Ni-based superalloy blade. Liquation occurs basically either by non-equilibrium interface melting below an alloy solidus or by equilibrium supersolidus melting. Refer to DS blade, liquation is normally attributed to the melt of low melting point eutectic in the parallel line-shape coarse grain boundaries. However, laser deposition is a typical rapid heating/ rapid cooling non-equilibrium process, while it is well know that rapid heating will increase the liquidus of an alloy to several tens of degrees or even over a hundred degrees. Therefore, it is not clear the real mechanism of liquation happened in DS blade during laser deposition, to clarify the liquation mechanism on DS blade is then vital for developing a practical laser deposition technology for crack-free rejuvenation. Based on systematic experimental results on laser deposition In 738 onto DS Ni-based substrate, this paper summarizes 4 type of boundary liquation and interface cracking phenomena, and approved that the liquation during laser deposition on DS substrate initiates from the melting of the periphery of the WC carbides in the heat affected zone, instead of the as-believed liquation initiation from the low melting point of eutectics in the grain boundary. The melting of the carbide periphery further develops, connects with each other, propagates along the grain boundary and finally forms a liquid film and then the cracking. Based on this understanding, three approaches were then proposed to achieve crack-free laser deposition rejuvenation for DS superalloy blades.